1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to an image forming method and an image forming apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, organic photoreceptors (OPCs) have been used for various image forming apparatuses such as copiers, printers, facsimiles and multi-functional apparatuses instead of inorganic photoreceptors because of having the following advantages over inorganic photoreceptors.    (1) good optical properties such that the photoreceptors have photosensitivity over a broad wavelength range and can absorb a large amount of light;    (2) good electric properties such as high photosensitivity and stable charging property;    (3) a wide material selectivity (i.e., various kinds of materials can be used for the photosensitive layer);    (4) good productivity;    (5) low costs; and    (6) little toxicity.
Recently, image forming apparatuses are required to have a small size and to produce images at a high speed without frequent maintenance operations, and therefore a need exists for a small-size photoreceptor having a good durability. In general, organic photoreceptors are soft because of having an outermost layer including a low molecular weight charge transport material and an inactive polymer. Therefore, when image forming operations such as charging, developing, transferring and cleaning operations are repeatedly performed on such organic photoreceptors, the surface of the photoreceptors can be easily abraded due to the mechanical stresses applied thereto.
In addition, in order to produce high quality images, the particle size of the toners used for forming visual images in image forming apparatus becomes smaller and smaller. In order to well remove residual toner particles on the surface of the photoreceptors of the image forming apparatuses, a cleaning blade having a high hardness is contacted with the surface of the photoreceptors at a high pressure. Thereby, abrasion of the surface of photoreceptors is accelerated.
Abrasion of the surface of the photoreceptors deteriorates the photosensitivity and charging properties of the photoreceptors, resulting in decrease of image density and formation of abnormal images such as background development in that background of images is soiled with toner particles. If local abrasion is caused (such as formation of scratches) to the photoreceptors, the photoreceptors produce streak images due to defective cleaning.
Therefore various attempts have been made to solve the abrasion problem of OPCs.
As one of the attempts, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 08-262779 (i.e., Japanese patent No. (hereinafter JP) 3262488) discloses a photoreceptor having a crosslinked outermost layer prepared by crosslinking a polyfunctional radically polymerizable monomer. It is described therein that the technique has advantages such that the resultant outermost layer has a dense three-dimensional network because monomers having a large number of functional groups cab be used; the crosslinked outermost layer can be rapidly prepared using light, heat and/or radiation; and the resultant crosslinked outermost layer hardly deteriorates the electric properties of the resultant photoreceptor because the crosslinking reaction can be performed without using acids and bases.
Further, in order to improve the electric properties of such a crosslinked outermost layer, JP-As 05-216249 (i.e., JP 3194392) and 2000-66425 have disclosed to prepare photoreceptors having an outermost layer which is prepared by using a charge transport material having a radically polymerizable monomer to fix a charge transport structure in the crosslinked network. It is described therein that by using this technique, a good combination of abrasion resistance and charge transportability can be imparted to the resultant photoreceptor, and the outermost layer has sufficient thickness tolerance.
Although such a crosslinked outermost layer prepared by using a radically polymerizable monomer has a good abrasion resistance because of having a highly crosslinked three dimensional network, the outermost layer typically has a large dielectric constant because a large number of polar groups are included therein. Therefore, a problem which occurs is that the electric properties (such as photosensitivity) of the photoreceptor deteriorate because the resistance of the layer decreases due to oxidation gasses generated by chargers and change of the environmental conditions such as temperature and humidity, resulting in deterioration of image qualities such as decrease of image density, formation of tailed images and deterioration of resolution.
In attempting to solve the problem, JP-A 2006-3863 discloses a technique in that a polyfunctional monomer, some of whose functional groups are substituted with alkyl groups, is used to introduce inactive groups in the crosslinked layer, to suppress change of the electric properties of the photoreceptor due to changes of environmental conditions. In addition, JP-As 2006-3863 and 05-173350 (i.e., JP 2896823) have disclosed techniques in that a bisphenol A-based difunctional monomer is used in combination with radically crosslinkable monomers to improve the environmental stability of the crosslinked outermost layer, and adhesion of the outermost layer to the lower layer on which the outermost layer is formed, to attempt to prevent change of image density and peeling of the outermost layer.
Thus, it has been attempted to develop photoreceptors with improved environmental stability and resistance to oxidation gasses using the above-mentioned techniques. However, when the number of functional groups are increased to impart high abrasion resistance to the resultant photoreceptor, a number of polar groups and unreacted functional groups are present in the resultant layer, resulting in deterioration of the environmental stability of the photoreceptor. In contrast, when the number of functional groups are decreased, the mechanical strength (i.e., abrasion resistance) of the resultant outermost layer deteriorates. Thus, the abrasion resistance and environmental stability establish a trade-off relationship, and therefore a photoreceptor having a good combination of abrasion resistance and environmental stability has not yet been provided.
Because of these reasons, a need exists for an electrophotographic photoreceptor having a good combination of environmental stability and abrasion resistance.